1. Field of the Invention
The present invention relates to gas sensors for detecting a concentration of a specified gas component contained in a detection target gas such as exhaust gas emitted from an internal combustion engine.
2. Description of the Related Art
In general, one or more gas sensors are installed in an exhaust gas system mounted to a motor vehicle in order to detect a concentration of a specified gas component such as oxygen gas contained in exhaust gas emitted from an internal combustion engine of a motor vehicle.
Such a gas sensor has a structure in which a gas sensor element is fixed in the inside of a cylindrical housing case. Such a gas sensor has another structure in which an insulator is fixed in the inside of the cylindrical housing case, and a gas sensor element is fixed in the inside of the insulator.
For example, a conventional patent document, Japanese patent laid open publication No. JP 2003-114210, discloses a structure of a conventional gas sensor having a filler portion which is formed between the cylindrical housing case and another component of the gas sensor, specifically, formed between the housing case and the gas sensor element or between the housing case and the insulator. Granulated powder of talc (or talcum powder) is used as such filler powder. Using such filler powder makes it possible to provide a necessary airtightness between the housing case and the gas sensor element or the insulator in order to separate a detection target gas (exhaust gas) from a reference gas (for example, the atmosphere).
However, a temperature of exhaust gas is more increased due to increasingly strict vehicle emission regulations. This often causes a problem, namely, decreases the airtightness in the space formed between the housing case and the gas sensor element or the insulator in the conventional gas sensor having the structure previously described. If the airtightness of the gas sensor is decreased, an output of the gas sensor is decreased. As a result, the gas sensor becomes inaccurate.
FIG. 7A is a view showing conventional granulated powder 90 composed of talc 901 having a thin-leaf shape (or a scale shape). FIG. 7B is a view showing a structure of a filler portion which is filled with granulated powder 90 composed of thin-leaf shaped grains of talc 901.
The inventors according to the present invention investigated and studied the following matters which cause the deterioration of the airtightness of the gas sensor. When the space or gap between the housing case and the gas sensor element or between the housing case and the insulator in the conventional gas sensor is filled with the granulated powder 90 by applying a predetermined pressure, the filler portion 9 composed of the talc has a high degree of orientation of not less than 90% as shown in FIG. 7B because the thin-leaf shaped grains of talc 901 slide relative to each other.
The filler portion 9 which is filled with talc of a high degree of orientation has a very small coefficient of thermal expansion in a radial direction of the gas sensor (which is perpendicular to an axial direction of the gas sensor element in which the gas sensor element or the insulator is inserted into the cylindrical housing case).
The conventional gas sensor having the filler portion 9 cannot adequately follow the repetition of thermal shock cycle test of expansion and contraction of each of the housing case, the gas sensor element, the insulator, etc. This deteriorates the airtightness of the filler portion 9 in the conventional gas sensor.
Further, voids 902 and gaps are generated in the filler portion 9 composed of the granulated powder 90 in the conventional gas sensor by the repetition of thermal test cycle of executing expansion and contraction. This makes the thin-leaf shaped grains of talc 901 in the filler portion 9 in the conventional gas sensor move easily when a force is applied to the conventional gas sensor in a direction which is different from an axial direction of the conventional gas sensor. As a result, this decreases the airtightness of the filler portion 9 in the conventional gas sensor.